Apparatus for analyzing samples using combined thermal wave and X-ray reflectance measurements

ABSTRACT

This invention provides a measurement device that includes both an X-ray reflectometer and a thermal or plasma wave measurement module for determining the characteristics of a sample. Preferably, these two measurement modules are combined into a unitary apparatus and arranged to be able to take measurements at the same location on the wafer. A processor will receive data from both modules and combine that data to resolve ambiguities about the characteristics of the sample. The processor can be part of the device or separate therefrom as long as the measurement data is transferred to the processor.

PRIORITY

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/189,334, which provisional application was filed onMar. 14, 2000 and is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

[0002] The present invention relates to the field of metrology tools formeasuring semiconductor wafers and, in particular, relates to a toolthat combines two complementary types of measurements into a single toolto reduce ambiguities in both types of measurements.

BACKGROUND OF THE INVENTION

[0003] The semiconductor industry has a continuing interest in measuringvarious thin film layers formed on semiconductor wafers. A number ofmetrology devices have been developed for making these measurements.

[0004] One example of such an apparatus is disclosed in PCT applicationWO/9902970, published Jan. 21, 1999. The assignee herein hascommercialized the device described in that patent application under thename OPTI-PROBE 5240. This device includes a number of measurementtechnologies. More specifically, the device includes a beam profileellipsometer (BPE) (see U.S. Pat. No. 5,181,080); a beam profilereflectometer (BPR) (see U.S. Pat. No. 4,999,014); relativelyconventional broad band (BB) and deep ultraviolet (DUV) spectrometers; aproprietary broad band spectroscopic ellipsometer (SE) (see U.S. Pat.No. 5,877,859) and an off-axis narrow band ellipsometer (see U.S. Pat.No. 5,798,837). All of the above-recited patents and PCT applicationsare incorporated herein by reference.

[0005] The above described system is particularly suited forcharacterizing relatively transparent films, such as silicon dioxide, onsemiconductors. This measurement system is somewhat less useful whenanalyzing opaque films such as metals.

[0006] An optical technique which is particularly suited for measuringthe thickness of very thin metal films is X-ray reflectometry. Using aprobe beam generated by a source of very short wavelength radiation,thin films can be analyzed which are opaque to both visible and UVwavelengths. One example of such a system is described in U.S. Pat. No.5,619,548, issued Apr. 8, 1997, and incorporated herein by reference.

[0007] In an X-ray reflectometer, a probe beam of X-ray radiation isdirected to impinge on the sample at an angle so that it is at leastpartially reflected. A sample may typically consist of a substratecovered by one or more thin metal layers. At very shallow angles, belowa critical angle (Ψ_(c)) (as measured between the surface of the sampleand the incoming ray), all of the X-ray radiation will be reflected.Typical incidence angles are very shallow, near grazing incidence,because the reflectivity falls very quickly as the angle is increasedabove the critical angle. As the angle of incidence of the incoming beamincreases, an increasing amount of radiation will be transmitted throughthe top metal layer and the amount of reflected light will be reduced.Some of the radiation transmitted through the metal layer(s) will reachthe interface between the metal film and the substrate and be reflectedfrom the substrate. The radiation reflected at the interfaces among themetal film layers and the substrate will interfere, giving rise to areflectivity curve showing interference effects. By analyzing thedependence of the reflectivity on the angle of incidence, one canindependently determine both the thickness and density of the thin filmlayers on the sample.

[0008] The added capability offered by an X-ray reflectometer has ledprior researchers to attempt to combine the measurements from an X-rayreflectometer with those of other optical measurement tools. Forexample, samples have been analyzed using a combination of grazing X-rayreflectometry and spectroscopic ellipsometry. (See, “A new versatiesystem for characterization of antireflective coatings using combinedspectroscopic ellipsometiy and grazing X-ray reflectance,” Boher, SPIE,Vol. 3741, page 104, May 1999.) Other researchers have proposedcombining X-ray reflectometry with infrared spectroscopy andtransmission spectroscopy. In addition, researchers have also discussedthe desirability of obtaining multiple separate measurements includingX-ray reflectometry, variable angle of incidence reflectometry and“mirage” style, photo-thermal measurements to evaluate a sample. (See,“Optical and X-ray characterization applied to multilayer reverseengineering,” Boudet, Optical Engineering, Vol. 37 (1), page 2175, July1998). In this paper, the authors used the photothermal method toanalyze losses from absorption.

[0009] The inventors herein have recognized that there are furtheradvantages to combining the measurements that can be obtained from X-rayreflectometry with measurements that can be obtained from a thermaland/or plasma wave analysis. A thermal and plasma wave metrology deviceis marketed by the assignee herein under the name of Therma-Probe. Thisdevice incorporates technology described in the following U.S. Pat. Nos.4,634,290; 4,646,088; 5,854,710 and 5,074,669. The latter patents areincorporated herein by reference.

[0010] In the basic device described in the patents, an intensitymodulated pump laser beam is focused on the sample surface forperiodically exciting the sample. In the case of a metal, thermal wavesare generated, while in a semiconductor, both thermal and plasma wavesare generated. These waves spread out from the pump beam spot andreflect and scatter off various features and interact with variousregions within the sample in a way which alters the flow of heat and/orplasma from the pump beam spot.

[0011] The presence of the thermal and plasma waves has a direct effecton the reflectivity at the surface of the sample. Features and regionsbelow the sample surface which alter the passage of the thermal andplasma waves will therefore alter the optical reflective patterns at thesurface of the sample. By monitoring the changes in magnitude and/orphase of the reflectivity of the sample at the surface, informationabout characteristics below the surface can be investigated.

[0012] In the basic device, a second laser is provided for generating aprobe beam of radiation. This probe beam is focused colinearly with thepump beam and reflects off the sample. A photodetector is provided formonitoring the periodic changes in the magnitude and phase of thereflected probe beam. The photodetector generates an output signal whichis proportional to the reflected power of the probe beam and istherefore indicative of the varying optical reflectivity of the samplesurface.

[0013] The output signal from the photodetector is filtered to isolatethe changes which are synchronous with the pump beam modulationfrequency. In the preferred embodiment, a lock-in detector is used tomonitor the magnitude and phase of the periodic reflectivity signal.This output signal is conventionally referred to as the modulatedoptical reflectivity (MOR) of the sample.

[0014] This system has been used successfully to measure ionimplantation levels in semiconductors. Such a system can also be used tomeasure thermal conductivity or thermal diffusion in a thin film layeron a sample (see U.S. Pat. No. 5,074,669, incorporated by reference).Such a system can also be used to measure characteristics of thinmetalized layers (see U.S. Pat. No. 5,978,074, incorporated byreference).

[0015] Other techniques besides modulated optical reflectivity detectioncan be used to monitor the effects of thermal and plasma wavepropagation in a sample. For example, various interferometry and mirageeffects techniques have been used. The broad scope of the subjectinvention includes these techniques as well. (See for example, U.S. Pat.No. 6,108,087).

[0016] Since an X-ray reflectometer measurement permits determination ofboth thickness and density independently, it has been recognized by theinventors herein that special benefits can be obtained by analyzing asample using a combination of X-ray reflectometry along with a thermaland plasma wave measurement technique. More specifically, if informationabout the thickness and/or density of the thin film can be obtainedusing an X-ray reflectometer measurement, the remaining variables (suchas the index of refraction and thermal conductivity) can be more easilydetermined with a thermal wave tool since one less variable needs to beresolved through the analysis of the data.

SUMMARY OF THE INVENTION

[0017] Accordingly it is an object of this invention to provide ameasurement device that includes both an X-ray reflectometer and athermal and plasma wave measurement module for determining thecharacteristics of a sample. Preferably, these two measurement modulesare combined into a unitary apparatus and arranged to be able to takemeasurements at the same location on the wafer. A processor will receivedata from both modules and combine that data to resolve ambiguitiesabout the characteristics of the sample. The processor can either bepart of the unitary apparatus or separate therefrom as long as themeasurement data is transferred to the processor. In either case, theapparatus is unitary so long as the X-ray reflectometer and the thermaland/or plasma wave measurement modules are combined into a singledevice.

BRIEF DESCRIPTION OF THE FIGURES

[0018]FIG. 1 is a composite block and schematic diagram of an apparatusfor carrying out the detection of thermal and plasma waves in accordancewith the subject invention.

[0019]FIG. 2 is a composite block and schematic diagram of an apparatusfor carrying out X-ray reflectometric measurements in accordance withthe subject invention.

[0020]FIG. 3 is a composite block and schematic diagram of a unitaryapparatus for carrying out both X-ray reflectometric measurements andthermal wave measurements in accordance with the subject invention

DETAILED DESCRIPTION OF THE INVENTION

[0021] The subject combination can be used effectively on samples withcomplex multilayer samples. For example, consider a silicon wafer uponwhich has been deposited a thin layer of tantalum nitride (250Angstroms) covered by a relatively thicker layer of copper (2000Angstroms or greater). Tantalum nitride is a relatively opaque metallicmaterial. For such a sample, an X-ray reflectometer measurement would beable to accurately determine the thickness of the thin intermediatelayer of tantalum nitride. This is so because the X-rays will penetratethe copper and reflect off the tantalum nitride. The interferencefringes can then be analyzed to determine layer thicknesses. Inaddition, the X-ray reflectance measurements can also provideinformation about the density of the top copper layer. However, theX-ray reflectance measurements can less easily determine the thicknessof the copper layer since any interference fringes associated with thatlayer would be too closely spaced together to allow resolution.Nonetheless, the thickness of the thicker copper layer can easily bemeasured using a thermal wave analysis. The tantalum nitride layer willessentially be invisible to the thermal wave analysis. By combining thetwo measurements in a single tool, the user can more readily obtain agreater amount of information about the multilayer structure.

[0022] In addition, thermal wave measurements can also be used todetermine the diffusivity of a layer. Diffusivity is a function of thethermal conductivity, density, and the specific heat of the material.For most materials, the specific heat does not vary significantly withina given layer. As noted above, X-ray reflectometry can be used todetermine the density of a layer. Thus, by combining X-ray reflectometrymeasurements (which enable a density analysis) with thermal wavemeasurements (which enable a diffusivity analysis), the thermalconductivity of the layer can be determined even if the thickness of thelayer is unknown. It is also possible to evaluate sheet resistance basedon the calculated thermal conductivity of the material.

[0023] Another advantage of the subject combination relates to productwafer measurement capability. It is known that the assignee'sTherma-Probe system can measure on product wafers with small featuresizes. In part this capability arises from the smallness of the focusedpump and probe beam spot size on the sample in conjunction with accuratewafer positioning controls. While the X-ray reflectometry spot tends tobe significantly larger than the thermal wave test spot, it has beenshown by the assignee herein that the X-ray interaction can also be usedon product wafers. This relationship is described in co-pendingapplication Ser. No. 09/629,407, filed Aug. 1, 2000, and incorporatedherein by reference. In brief, it has been found that the X-rays do notscatter very strongly when interacting with structures found on productwafers. Therefore, using a proper analysis an X-ray reflectometer cancharacterize blanket film structures deposited on a product wafer almostas easily as if the wafer were a test wafer. Since both of thesetechnologies can provide information about product wafers, thecombination can be used to further analyze structures on product wafersthereby minimizing the need for test wafers.

[0024] Providing multiple measurement tools in a single device has addedbenefits. For example, it is possible to arrange the optical systems tomeasure on the same point on the wafer without moving the wafer. Inaddition, a single tool has a smaller footprint and therefore takes upless floor space in the semiconductor fabrication facility. By combiningtechnologies in a single tool, costs can be reduced by eliminatingduplicate subsystems such as wafer handlers and computers. Finally, thecombination can simplify and streamline decision making since theinformation from the two measurement modalities can be coordinatedinstead of producing conflicting results as in the prior art when twoseparate devices might be used.

[0025] Further analytical capability can be obtained if the device isarranged to include additional measurement modalities. Such additionalmeasurement modalities can include one or more optical metrology devicesof the type found in the assignee's OPTI-PROBE 5240, discussed above.

[0026] Referring to FIG. 1, there is illustrated an apparatus 20 formonitoring thermal and plasma waves. The apparatus of FIG. 1 illustratesonly the basic elements. Those skilled in the art will understand that acommercial devices will be more complex. More details of a thermal wavesystem are disclosed in U.S. Pat. No. 5,978,074, cited above.

[0027] As seen in FIG. 1, a sample 22 rests on a platform 24. Platform24 is capable of movement in two orthogonal directions in a manner suchthat the sample can be rastered with respect to the heating and probebeams of the subject invention.

[0028] The means for generating thermal and plasma waves includes alaser 30 emitting a beam 34 which is intensity modulated by modulator32. In the preferred embodiment, beam 34 is focused on the surface ofthe sample by a microscopic objective 38. Beam 34 is intended toperiodically excite the sample surface. This periodic excitation is thesource of thermal and plasma waves that propagate outwardly from thecenter of the beam. The plasma and thermal waves interact with sampleboundaries and barriers in a manner that is mathematically equivalent toscattering and reflection of conventional propagating waves. Anyfeatures on or beneath the surface of the sample that have thermal orplasma diffusion characteristics different from their surroundings willreflect and scatter thermal and plasma waves and thus become visible tothese waves.

[0029] The detection system includes a laser 50 for emitting a probebeam 52. Probe beam 52 is directed onto a region of the sample surfacethat has been periodically heated by the modulated energy beam 34. Probebeam 52 is first passed through a polarizing splitter 54 oriented in amanner such as to let the coherent light emanating from laser 50 to passfreely therethrough. The splitter will, however, deflect all light whosephase has been rotated through 90 degrees relative to beam 52. Thereason for this arrangement will become apparent below.

[0030] Light probe beam 52 is then passed through a 1/4λ-waveplate 55.Waveplate 55 functions to rotate the phase of the probe beam by 45degrees. As can be appreciated, on the return path of the beam, thewaveplate will rotate the phase of the beam another 45 degrees so thatwhen it reaches splitter 54 the phase of the beam will have been rotateda total of 90 degrees from the incoming orientation. By thisarrangement, the splitter 54 will deflect the retro-flected light beamup to detector 56.

[0031] After the probe beam 52 initially passes through waveplate 55, itis reflected downwardly by dichroic mirror 36. The dichroic mirror isselected to be transparent to the pump beam wavelength and reflective ofthe probe beam wavelength. In the preferred embodiment, the pump beamand the probe beam are aligned in such a manner that they are directedin a coincident manner down through lens 38 and focused at the same spoton the surface of the sample. By focusing the pump and probe beams atthe same spot, the maximum signal output can be achieved.

[0032] The probe beam is reflected back up to the dichroic mirror whereit is, in turn, reflected back along the incoming path and through the1/4λ-waveplate 55. As discussed above, waveplate 55 rotates the phase ofthe probe beam by another 45 degrees such that when the beam reachessplitter 54, its phase has been rotated 90 degrees with respect to theoriginal beam. Accordingly, this splitter will deflect theretro-reflected probe beam upwardly towards detector 56.

[0033] Since intensity variations of a radiation beam are to bedetected, a standard photodetector may be employed as a sensingmechanism. The intensity variations which are measured are then suppliedas an output signal to a processor for deriving the data on the thermaland plasma waves based on the changing surface temperature conditions asindicated by the changing output signal.

[0034] The operation of processor 58 is dependent on the type of testingconfiguration which is utilized. In all cases, the processor is designedto evaluate the intensity changes of the incoming probe beam which arethe result of the periodic reflectivity changes caused by the periodicheating on the sample. These periodic intensity changes are filtered toproduce a signal which may be evaluated. Details of a suitable detectorand processor arrangement are disclosed in U.S. Pat. No. 5,978,074. Thelatter patent also discloses how thermal waves can be detected bymonitoring the periodic angular deflections of a probe beam. As notedabove, other techniques are known for thermal wave measurementsincluding the mirage technique and interferometric techniques. (See forexample, the articles cited of record in U.S. Pat. No. 4,521,118)

[0035] A preferred XRR technique for use in the subject combination isdescribed in U.S. Pat. No. 5,619,548, issued Apr. 8, 1997, which ishereby incorporated by reference in its entirety. FIG. 2 illustrates thebasic components for this technique. More details of a suitable XRRsystem can be found in U.S. Ser. No. 09/527,389, filed Mar. 16, 2000.

[0036] Referring to FIG. 2, the X-ray scattering system includes anX-ray source 31 producing an X-ray bundle 33 that comprises a pluralityof X-rays shown as 35 a, 35 b, and 35 c. An X-ray reflector 37 is placedin the path of the X-ray bundle 33. The reflector 37 directs the X-raybundle 33 onto a test sample 39 held in a fixed position by a stage 45,and typically including a thin film layer 41 disposed on a substrate 43.Accordingly, a plurality of reflected X-rays, 57 a, 57 b, and 57 c(forming bundle 46) concurrently illuminate the thin film layer 41 ofthe test sample 39 at different angles of incidence. The X-ray reflector37 is preferably a monochromator. The diffraction of the incident bundleof X-rays 33 within the single-crystal monochromator allows only anarrow band of the incident wavelength spectrum to reach the sample 39,such that the Bragg condition is satisfied for this narrow band. As aresult, the plurality of X-rays 57 a, 57 b, and 57 c, which are directedonto the test sample 39, are also monochromatic. A detector 47 ispositioned to sense X-rays reflected from the test sample 39 and toproduce signals corresponding to the intensities and angles of incidenceof the sensed X-rays. A processor 60 is connected to the detector toreceive signals produced by the detector in order to determine variousproperties of the structure of the thin film layer, including thickness,density and surface roughness.

[0037] In a basic system, a probe beam of X-ray radiation is directed tostrike the sample at an angle selected so that it is at least partiallyreflected. A sample may typically consist of a substrate covered by oneor more thin metal layers. At very shallow angles, below a criticalangle (Ψ_(c)) (as measured between the surface of the sample and theincoming ray), all the X-ray radiation will be reflected. As the angleof incidence of the incoming beam increases with respect to the samplesurface, an increasing amount of radiation will be transmitted throughthe top metal layer and the amount of reflected light will be reduced.Some of the radiation transmitted through the metal layer(s) will reachthe interface between the metal film and the substrate and be reflectedoff the substrate.

[0038] The radiation reflected at the interfaces among the metal filmlayers and the substrate will interfere, giving rise to a reflectivitycurve showing interference effects.

[0039] For a given sample of thin films, X-ray reflectivity can bedetermined using a Fresnel equation modeling as a function principallyof X-ray wavelength (λ), angle of incidence, and the thicknesses andoptical properties of the materials making up the layers. Typically thecritical angle at which total reflection occurs is quite small(˜0.1-0.5°). Because reflectivity falls very quickly as the angle ofincidence is increased above the critical angle, small angle X-rayreflection is experimentally important. Under a small angleapproximation (sin ψ≅ψ), a recursive formula for the X-ray reflectivityat an interface between a layer n−1 and a layer n is given by${R_{{n - 1},n} = {a_{n - 1}^{4}\left( \frac{R_{n,{n + 1}} + F_{{n - 1},n}}{R_{n,{n + 1}} + F_{{n - 1},n} + 1} \right)}},$

[0040] where

[0041] F_(n−1,n)=(f_(n−1)−f_(n))/(f_(n−1)+f_(n)),

[0042] and where a_(n)=exp((−iπ/λ)f_(n)d_(n)).

[0043] Here d_(n) is the thickness of layer n and ψ_(c)(n) is thecritical angle at which total reflection occurs for X-rays of wavelengthλ incident on material of layer n.

[0044] The f_(n) are given by

f _(n) =A _(n) −iB _(n),

[0045] where $\begin{matrix}{A_{n} = \quad {\left( {1/\sqrt{2}} \right)\left( {\left\{ {\left\lbrack {\psi^{2} - {\psi_{c}^{2}(n)}} \right\rbrack^{2} + {4\beta_{n}^{2}}} \right\}^{1/2} + \left\lbrack {\psi^{2} - {\psi_{c}^{2}(n)}} \right\rbrack} \right)^{1/2}}} \\{{B_{n} = \quad {\left( {1/\sqrt{2}} \right)\left( {\left\{ {\left\lbrack {\psi^{2} - {\psi_{c}^{2}(n)}} \right\rbrack^{2} + {4\beta_{n}^{2}}} \right\}^{1/2} - \left\lbrack {\psi^{2} - {\psi_{c}^{2}(n)}} \right\rbrack} \right)^{1/2}}},}\end{matrix}$

[0046] and where

[0047] β_(n)=λμ_(n)/4π, ψ is the angle of incidence of the X-rays, andμ_(n) is the linear absorption coefficient of the layer n.

[0048] These recursive equations are solved by setting R_(N, N+1) equalto 0 with layer n=N corresponding to the substrate and carrying out theresulting recursive calculations, starting at the bottom of the thinfilm stack. With layer n=1 corresponding to the vacuum, the product(|R_(1,2)|²) of R_(1,2) with its complex conjugate gives the ratio ofthe reflected X-ray intensity to incident X-ray intensity.

[0049] The theoretical modeling of X-ray reflection based on theclassical Fresnel equations, as well as complications from the width ofinterfaces and microscopic surface roughness, are discussed in greaterdetail in the following references, each of which is hereby incorporatedby reference in its entirety: L. G. Parratt, Phys. Rev. 95, 359 (1954);C. A. Lucas et al., J. Appl. Phys. 63, 1936 (1988); M. Toney, S.Brennan, J. Appl. Phys. 66, 1861 (1989).

[0050] One approach to measuring the film thicknesses of patternedsemiconductor wafers using XRR relies on the recognition that themeasured X-ray reflection curve can be attributed primarily to thethicknesses of the layers rather than the structure of the pattern. Thewavelength of the X-rays used in the XRR measurement is on the order ofa few angstroms. Compared to the feature size of the patterned wafers,which is on the order of 10,000 angstroms, the wavelength is very small.Therefore interference effects from the structure of the pattern itselfare not important. The most noticeable effect is that the reflectedX-ray intensity may be generally reduced since the portion of the lightthat is incident onto the sides and bottoms of the recesses contributesless to the reflected signal. When the depth of the recesses is largecompared to the thickness of the layers being measured, one sees onlyminor changes in the X-ray reflectivity curve beyond the reduction inoverall intensity.

[0051] As used herein, “patterned wafer” or “patterned semiconductorwafer” means a semiconductor wafer whose surface bears an artificialpattern whose features are small in size relative to the spot size ofthe X-ray probe beam. As noted above, typically, the measurement spotsize for the probe beam is one millimeter or larger, while the featuresof the pattern are on the order of one micron in size, and even the testsites on a patterned wafer have dimensions typically smaller than 100microns. Thus, there is typically at least an order of magnitudeseparating the X-ray probe beam spot size and the size of even the testsites on the patterned wafer.

[0052] Because of the similarity in shape of the X-ray reflectancecurves, analysis of the X-ray reflectivity curve for a patterned wafercan be greatly simplified through comparison with measurements made onan unpatterned wafer having similar layers. The unpatterned comparisonwafer could be simply an unpatterned region on the patterned wafer,which unpatterned region underwent similar deposition as the patternedregion.

[0053] In the case of the patterned wafer data, a simple transformationis applied based on the close resemblance of the patterned waferreflectivity curve RP(θ) and the unpatterned wafer reflectivity curveRU(θ). (Here θ is the angle of incidence, but other dependent variables,such as the wave vector transfer, could also be used.) A transformationfunction T(θ) is chosen such that RP(θ)×T(θ) closely approximates RU(θ).The resemblance of RP(θ) and RU(θ) is such that T(θ) may appropriatelybe a simple linear function of θ. However, more complex functions couldalso be chosen so that, for example, T(θ) could appropriately be aquadratic or cubic function of θ or a “splicing” of such functions fordifferent portions of the angular spectrum.

[0054] Using a simple linear transformation function, T(θ), the data forthe patterned wafer can be transformed. The same Fresnel equationmodeling that are applied to an unpatterned wafer can be applied to thetransformed reflectivity data to find the layer thicknesses for apatterned wafer. The necessary parameters can be found through aniterative nonlinear least squares optimization technique such as thewell-known Marquardt-Levenberg algorithm. A suitable iterativeoptimization technique for this purpose is described in “MultiparameterMeasurements of Thin Films Using Beam-Profile Reflectivity,” Fanton etaL, Journal of Applied Physics, Vol. 73, No. 11. p. 7035 (1993) and“Simultaneous Measurement of Six Layers in a Silicon on Insulator FilmStack Using Spectrophotometry and Beam Profile Reflectometry,” Leng etal., Journal of Applied Physics, Vol. 81, No. 8, p. 3570 (1997). Thesetwo articles are hereby incorporated by reference in their entireties.

[0055] Once the layer thickness is determined, one can then analyze thefull R-Ψ curve and obtain values for density and surface and interfaceroughness.

[0056] Another approach to finding the layer thicknesses for anunpafterned wafer is to use a Fourier transform analysis. Fouriertransform analysis was applied to find layer thicknesses of polymersystems in Seeck et al., AppI. Phys. Left. 76, 2713 (2000), herebyincorporated by reference in its entirety.

[0057] In another approach, when different fringe regimes arediscernible in the data, the thicknesses of the metal films on apatterned wafer can be determined by reference to a modified Braggequation as follows:

sin²Ψ_(i)=sin²Ψ_(c)+(i+{fraction (1/2)}) ² (λ/2d)²

[0058] where “Ψ_(i)” is the angle at which there is a fringe maximum,Ψ_(c) is the critical angle, i is a positive integer with values 1, 2,3, . . . , λ is the X-ray wavelength, and d is the layer thickness.

[0059] Since Ψ_(i) and Ψ_(c) are very small angles, and since themodified Bragg equation must be valid for all critical angles, includingΨ_(c)=0, under this approximation the angular spacing between adjacentinterference fringes is a constant for a given thickness d and is givenby:

ΔΨ=λ/2d

[0060] Using this approach, a thickness d(ΔΨ)=λ/(2ΔΨ) can be associatedwith each fringe spacing in the curve. Since the approximative Braggequation becomes more valid as the angle of incidence increases, anasymptotic analysis can be applied to find the true thickness d byplotting the values for d(ΔΨ) as a function of increasing 0 andextrapolating the asymptote.

[0061] In the preferred embodiment, the two different measurementmodalities represented by FIGS. 1 and 2 would be arranged so that theboth measurements could easily be made in the same region of the sample.Typically, the measurements would be made sequentially. Given thegeometries of the techniques, it would be possible to arrange theoptical elements so that only a single stage is necessary. Moreparticularly, and as shown in simplified form in FIG. 3, the pump andprobe beams (34, 52) of the modulated optical reflectivity system can bedirected normal to the sample surface. In contrast, the X-ray probe beam(bundle 46) is directed at near grazing incidence to the sample, thuspermitting both optical systems to be arranged to measure essentiallythe same region on the sample. The data from both measurement modulescan be supplied to a common processor 60 which can integrated with thesame device or located remotely from the device. Using data taken byboth modules from the same region on the sample will improve measurementaccuracy.

[0062] The scope of the present invention is meant to be that set forthin the claims that follow and equivalents thereof, and is not limited toany of the specific embodiments described above.

What is claimed is:
 1. A unitary apparatus for evaluating a samplecomprising: (a) a first measurement module including: (i) means forinducing a periodic localized excitation at the surface of the sample;(ii) means for directing a first probe beam of radiation within aportion of the area periodically excited in a manner such that the firstprobe beam reflects from the surface of the sample; (iii) means formeasuring the periodic variations of the reflected first probe beaminduced by said periodic excitation to generate first output signals;(b) a second measurement module including: (i) means for generating asecond probe beam, said second probe beam having X-ray wavelengths; (ii)means for directing said second probe beam onto the surface of saidsample; (iii) a detector for measuring the intensity of X-rays reflectedfrom said sample to generate second output signals; and (c) a processorfor evaluating the sample based on a combination of the first and secondoutput signals.
 2. An apparatus as recited in claim 1 wherein said meansfor measuring the intensity of X-rays includes a photodiode detector. 3.An apparatus as recited in claim 1 wherein said first probe beam isgenerated by a laser.
 4. An apparatus as recited in claim 1 wherein saidmeans for measuring the first probe beam measures periodic changes inthe magnitude or phase of the beam.
 5. A method of evaluating a samplecomprising the steps of: (a) obtaining a first set of measurements by:(i) inducing a periodic localized excitation on the surface of thesample; (ii) directing a first probe beam of radiation within a portionof the area periodically excited in a manner such that the first probebeam reflects from the surface of the sample; and (iii) measuring theintensity variations of the reflected first probe beam resulting fromperiodic changes of the sample induced by said periodic excitation togenerate first output signals; and (b) obtaining a second set ofmeasurements by: (i) generating a second probe beam of X-rays; (ii)directing said second probe beam-onto the surface of said sample; (iii)measuring the intensity of X-rays as reflected from said sample togenerate second output signals; and (c) evaluating the sample based on acombination of the first and second output signals.
 6. A method asrecited in claim 5 wherein said step of evaluating the sample includesusing either of the first or second output signals to characterize oneparameter of the sample and wherein the other output signals are used tofurther characterize the sample with said one parameter being treated asa known parameter.
 7. A method as recited in claim 5 wherein said stepof evaluating the sample includes using the second output signal tocharacterize the density of a sample layer and wherein the first outputsignals are used to further characterize the sample with said layerdensity being treated as a known parameter.
 8. A method as recited inclaim 5 wherein said step of evaluating the sample includes using thesecond output signal to characterize the thickness of a sample layer andwherein the first output signals are used to further characterize thesample with said layer thickness being treated as a known parameter. 9.A method as recited in claim 5 wherein said measuring of the intensityof X-rays includes using a photodiode detector.
 10. A method as recitedin claim 5 wherein said first probe beam of radiation is generated by alaser.
 11. A method as recited in claim 5 wherein the periodicvariations in the magnitude and/or phase of the first probe beam aremeasured.
 12. A method of evaluating characteristics of a samplecomprising the steps of: periodically exciting a region on the surfaceof the sample; monitoring the modulated optical reflectivity induced bysaid periodic excitation and generating first output signals in responsethereto; directing a probe beam of X-ray radiation onto the same regionon the sample surface; monitoring the non-modulated reflected power ofthe X-ray probe beam and generating second output signals in responsethereto; and evaluating the characteristics of the sample based on acombination of the first and second output signals.
 13. A method asrecited in claim 12 wherein said step of evaluating the characteristicsof the sample includes using either of the first or second outputsignals to characterize one parameter of the sample and wherein theother output signals are used to further characterize the sample withsaid one parameter being treated as a known parameter.
 14. A unitaryapparatus for evaluating characteristics of a sample comprising: anintensity modulated excitation source for periodically exciting a regionon the surface of the sample; means for monitoring the modulated opticalreflectivity induced by said periodic excitation and generating firstoutput signals in response thereto; means for obtaining X-rayreflectivity information from the same region on the sample surface andgenerating second output signals in response thereto; and a processorfor evaluating the characteristics of the sample based on a combinationof the first and second output signals.
 15. An apparatus as recited inclaim 14 wherein the processor uses either of the first or second outputsignals to characterize one parameter of the sample and wherein theother output signals are used to further characterize the sample withsaid one parameter being treated as a known parameter.
 16. A unitaryapparatus for evaluating characteristics of a sample comprising: anintensity modulated pump beam directed to the sample for periodicallyexciting a region on the surface of the sample; a first probe beamdirected to reflect off the periodically excited region; a firstdetection module having a photodetector for monitoring the modulatedchanges in the reflected first probe beam induced by said periodicexcitation and generating first output signals in response thereto; asecond probe beam of X-rays directed to reflect off the same region onthe sample surface; a second detection module for monitoring thenon-modulated reflected power of the second probe beam and generatingsecond output signals in response thereto; and a processor forevaluating the characteristics of the sample based on a combination ofthe first and second output signals.
 17. An apparatus as recited inclaim 16 wherein the first detection module monitors the modulatedvariations in the magnitude and/or phase of the first probe beam.
 18. Anapparatus as recited in claim 16 wherein the processor uses either ofthe first or second output signals to characterize one parameter of thesample and wherein the other output signals are used to furthercharacterize the sample with said one parameter being treated as a knownparameter.